In general, an organic electroluminescent device (hereinafter referred to as organic EL device) includes a light-emitting layer and a pair of counter electrodes interposing the light-emitting layer therebetween in its simplest structure. That is, the organic EL device uses the phenomenon that, when an electric field is applied between both the electrodes, electrons are injected from a cathode and holes are injected from an anode, and each electron and each hole recombine in the light-emitting layer to emit light.
In recent years, progress has been made in developing an organic EL device using an organic thin film. In order to enhance luminous efficiency particularly, optimization of kinds of electrodes has been attempted for the purpose of improving efficiency of injection of carriers from the electrodes. As a result, there has been developed a device in which a hole-transporting layer formed of an aromatic diamine and a light-emitting layer formed of an 8-hydroxyquinoline aluminum complex (hereinafter referred to as Alq3) are formed between electrodes as thin films, resulting in a significant improvement in luminous efficiency, as compared to conventional devices in which a single crystal of anthracene molecules or the like is used. Thus, development of the above-mentioned organic EL device has been promoted in order to accomplish its practical application to a high-performance flat panel having features such as self-luminescence and rapid response.
Further, studies have been made on using phosphorescent light rather than fluorescent light as an attempt to raise luminous efficiency of a device. Many kinds of devices including the above-mentioned device in which a hole-transporting layer formed of an aromatic diamine and a light-emitting layer formed of Alq3 are formed emit light by using fluorescent light emission. However, by using phosphorescent light emission, that is, by using light emission from a triplet excited state, luminous efficiency is expected to be improved by about three times to four times, as compared to the case of using conventional devices in which fluorescent light (singlet) is used. In order to accomplish this purpose, studies have been made on adopting a coumarin derivative or a benzophenone derivative as a light-emitting layer, but extremely low luminance has only been provided. Further, studies have been made on using a europium complex as an attempt to use a triplet state, but highly efficient light emission has not been accomplished. In recent years, many studies centered on an organic metal complex such as an iridium complex have been made, as disclosed in Patent Literature 1, for the purpose of attaining high luminous efficiency and a long lifetime.